Process for preparing a glass-ceramic body

ABSTRACT

Process for preparing glass-ceramic body including the steps of providing a basic glass body and subjecting the basic glass body to a thermal treatment whereby a crystalline phase embedded in a glass matrix is formed. The basic glass body is made of a composition comprising 65 to 72 wt-% SiO 2 , at least 10.1 wt-% Li 2 O and at least 10.1 wt-% Al 2 O 3  based on the total weight of the composition, the proportion of Li 2 O to Al 2 O 3  being from 1:1 to 1.5:1. The thermal treatment involves a nucleation step followed by several crystallization steps at different temperatures, whereby at least two different crystalline phases are formed.

FIELD OF THE INVENTION

The present invention relates to a process for preparing a glass-ceramicbody, to a glass composition for said process, as well as to aglass-ceramic body obtainable by said process and to the use of saidglass-ceramic body for a dental restoration.

BACKGROUND

Glass-ceramic materials comprise an amorphous (glass) phase and one ormore crystalline (ceramic) phases embedded in the amorphous phase. Dueto the presence of both an amorphous and a crystalline phase,glass-ceramics share many properties with both glasses and ceramics.They are used in a variety of different technical fields, for example ascooktops, cookware and bakeware, as a substrate for magnetic disks or ashigh performance reflectors for digital projectors.

Glass-ceramics are of particular interest in the field of restorativedentistry, in which the need for prostheses that, in terms offunctionality and appearance, would perform exactly like their naturalcounterparts has been expressed.

Conventionally, dental restorations have been prepared according to the“porcelain fused to metal” (PFM) method in which the supporting metalframework is used in conjunction with a veneering layer of a ceramicmaterial which makes up the colour of the prosthesis. The preparation ofrestorations according to this method implies many manufacturing stepsand is hence laborious.

The PFM method has been developed further by replacing the metallicframework by a non-metallic inorganic framework. In this regard, afeldspathic glass filled with alumina particles has been proposed.Further development has led to substituting an opaque ceramic frameworkfor the alumina-reinforced glass.

Dental crowns and bridges are today mostly manufactured by CAD/CAMtechnologies, which are increasingly gaining importance. The fabricationprocess comprises two decisive stages: a computer-aided design of therestoration and its computer-aided milling. In the stage of milling, therestoration is machined out of a blank.

DE-A-19750794 has proposed a process for preparing a lithium disilicateglass product suitable for the use as a dental product. The process isaiming at a high chemical stability, a high translucency and goodmechanical properties of the product. Due to the high strength andtoughness obtained, the machining of the material results, however, in avery high wear of the machining tools and very long processing times.Furthermore, restorations prepared according to this technique show onlya poor strength when their thickness falls within a range of only a fewhundreds of micrometers.

U.S. Pat. No. 7,452,836 relates to a process for providing aglass-ceramic which has metastable lithium metasilicate (Li₂SiO₃) asmain crystalline phase. This lithium metasilicate glass-ceramic hasmechanical properties allowing it to be easily machined into the shapeof even complicated dental restorations without undue wear of tools. Itcan be converted by further heat treatment into a lithium disilicateglass-ceramic with very good mechanical properties and translucency.

Although U.S. Pat. No. 7,452,836 allows for achieving materials having aflexural strength which might be sufficient for the restoration ofmultiple missing teeth in the front (for example 3-unit bridges), itsstrength is still not sufficient for posterior bridges or largerestorations.

Aiming not only at an improvement in mechanical properties, but also ata highly aesthetical appearance, a material having an internal structuremimicking the structure of a natural tooth would be highly appreciated.

Natural teeth consist of a hard, inert and acellular enamel supported bythe less mineralized, more resilient and vital hard tissue dentin.Because of its exceptionally high mineral content, enamel is a brittletissue unable to withstand the forces of mastication without fractureunless it has the support of the more resilient dentin.

Enamel and dentin do not only differ in their mechanical properties,namely their compressive strength, elastic modulus and coefficient ofthermal expansion, but also in their appearance. Whereas enamel istranslucent and varies in colour from light yellow to gray-white, dentinis yellow. In a natural tooth, the thickness of enamel varies from amaximum of approximately 2.5 mm to a fraction thereof. This variationinfluences the tooth's appearance because the underlying dentin is seenthrough the thinner enamel region, whereas it gradually fades outtowards thicker ones.

In summary, a natural tooth has thus an inhomogeneous structuredifferent than in the glass-ceramic of U.S. Pat. No. 7,452,835, in whichcrystals are grown throughout the whole volume without any spatialorder. In contrast to a natural tooth, which exhibits a differentcomposition and structure in different parts, be it in the dentin or theenamel part, a restoration made of the material according to U.S. Pat.No. 7,452,835 is with respect to the material constitution ratherhomogeneous and does not comprise regions of different constitutionslike the natural counterpart does. A natural tooth can thus not bemimicked perfectly by the material according to U.S. Pat. No. 7,452,835.

Biocompatible, highly aesthetical and robust materials with an internalstructure mimicking that of a natural tooth for a single toothreplacement (crowns) and for a prosthesis formed by two or more crowns(bridges) supported by modified natural teeth are, however, of paramountimportance in the field of restorative dentistry. Further, as moredental laboratories adopt CAD/CAM devices, laboratory-generated CAD/CAMprostheses are expected to rise significantly in the decades ahead. Thisevolution poses an additional requirement to materials for thefabrication of restoration viz. CAD/CAM machining at affordable costs.

A method for manufacturing prostheses from a blank comprising at leastone layer of high abrasive resistance, at least one layer of highflexural strength and at least one layer of lower hardness and strengthis disclosed in U.S. Pat. No. 5,939,211. During the milling of therestoration, material removal is performed in such a manner that layerswith high strength constitute a reinforcing structure.

Based on the finding that a so-called functionally graded material canlead to an improved resistance in contact damages, US 2008/0213727proposes a process for providing a functionally graded materialincluding infiltrating top and bottom ceramic surfaces with glass. Theresulting structure comprises an outer (aesthetic) surface residualglass layer, a graded glass-ceramic layer and a dense interior ceramic.

Further, WO 2010/010082 aims at a material mimicking the colourgradients in a natural tooth and relates to a form-stabilized materialcomprising a first component and a second component, the secondcomponent having a different pigmentation than the first component andbeing disposed in the first component such that the boundary surfacebetween the components represents a spatially curved surface.

In particular regarding U.S. Pat. No. 5,939,211 and WO 2010/010082, thepresence of physically distinct component layers and thus of aninterface between different components can have an impact on the overallstability of the dental restoration. Also, the processes according tothese documents are relatively laborious.

The technique according to US 2008/0213727 allows a gradient of only avery small thickness to be formed. In addition, the gradient is confinedto the surface area of the material; the formation of a gradient withinthe bulk of the material remote from the surface is however not possibleaccording to US 2008/0213727.

SUMMARY OF THE INVENTION

In general, it would be highly desirable to provide a glass-ceramicbody, the properties of which can be adjusted to the actual need in asimple and straightforward manner.

In particular for achieving dental restorations, with both highaesthetics and superior mechanical properties, a glass-ceramic bodywould be desirable for which different mechanical and optical propertiescan be obtained at different regions of one and the same body.

More particularly and in view of the drawbacks of US 2008/0213727, aglass-ceramic body would be desirable comprising different crystallinephases changing from one region to another in a gradual manner and notbeing locally limited to specific areas of the material, thus being ableto mimic the structure of a natural tooth.

The object of the present invention is thus to provide such aglass-ceramic body in a simple and straightforward manner.

According to first aspect, the present invention relates to a processfor preparing a glass-ceramic body comprising the steps of providing abasic glass body and subjecting the basic glass body to a thermaltreatment whereby a crystalline phase embedded in a glass matrix isformed.

According to the process of the invention, the basic glass body is madeof a composition comprising 65 to 72 wt-% SiO₂, at least 10.1 wt-% Li₂Oand at least 10.1 wt-% Al₂O₃ based on the total weight of thecomposition. Preferably, the proportion of Li₂O to Al₂O₃ is at least1:1, more preferably about 3:2.

According to a specific embodiment, the proportion of Li₂O to Al₂O₃ isfrom 1:1 to 1.5:1.

The thermal treatment involves a nucleation step followed by a firstcrystallization step at a first temperature range and a secondcrystallization step at a second temperature range different from thefirst temperature range. Thereby, at least two crystalline phasesdifferent from each other are formed.

It has surprisingly been found that by thermally treating a glass bodyaccording to the present invention, not only different crystallinephases can be formed, but that the type of the crystalline phases aswell as their proportion can be controlled and thus adjusted to theactual need.

In particular, different crystalline phases can be formed individuallyor together by applying the required heating for the crystallizationsteps in a focused manner. Thus, different crystalline phases can beformed in different regions of one and the same body.

Consequently, the present invention allows for achieving a structuredglass-ceramic body, i.e. a glass-ceramic body having crystalline phasesdiffering from region to region. In this regard, the present inventionfurther allows different crystalline phases to be achieved changing fromone region to another in a gradual manner. This has importantimplications in particular regarding the mechanical and opticalcharacteristics of the glass-ceramic body, more particularly in view ofa use of the body for a dental restoration, as will be discussed indetail below.

As mentioned, the thermal treatment according to the process of theinvention comprises a nucleation step prior to the crystallizationsteps. By the nucleation step, crystallization nuclei are formed. Forthe nucleation step, the basic glass body, i.e. the “starting glass”, isheated to a temperature of 500° C. to 570° C., which is slightly abovethe glass transition temperature, and a dwell of up to 3 hours isobserved. After this treatment, the nucleated glass does not show anysignificant difference in appearance from the starting glass. Followingthe nucleation step, the nucleated glass body is heated to highertemperatures and again dwells are observed at specific temperatures(crystallization steps). This treatment leads to differences in both themechanical properties and appearance of the glass body.

In summary, the process of the present invention, thus, comprises anucleation step at a first temperature range, after which no significantdifference in mechanical properties and appearance from the basic glassbody is observed, followed by at least two crystallization steps, eachat a temperature range higher than the one of the nucleation step, saidcrystallization steps leading to the formation of at least two differentcrystalline phases and, thus, to a glass-ceramic body having mechanicalproperties and an appearance different from the one of the basic glassbody.

The crystallization steps, thus, differ from the nucleation step in thatthey are performed at higher temperatures and that they go along with achange in both the mechanical properties and appearance of the material.

The thermal treatment of the present invention involving twocrystallization steps is different from the one described in DE 10 2007011 337 relating to veneer ceramics for dental restorations, the processof the latter comprising merely one crystallization step. The sameapplies to WO 00/34196, which relates to glass-ceramics useful in thefabrication of single and multi-unit dental restorations and whichdescribes a process of manufacture comprising merely one crystal growthstep after a nucleation step. Also, U.S. Pat. No. 6,514,890 describes aprocess comprising a single crystallization step after a nucleationstep, said document being—irrespective of the fact that a differentglass composition is used—thus different from the present invention.

Specifically, the basic glass body is heated to a temperature in therange from 500° C. to 570° C., more specifically from 530° C. to 570°C., for the nucleation step, followed by at least two crystallizationsteps selected from the ranges from 620° C. to 680° C., from 800° C. to820° C. and from 825° C. to 830° C., depending on the desiredcrystalline phases to be formed and their proportions in relation toeach other and to the amorphous phase. The duration for any of thesesteps typically vary in the range from 30 minutes to 10 hours, alsodepending on the desired crystalline phases to be formed and theirproportions.

It is understood that the process of the present invention can compriseone or more crystallization steps in addition to the first and thesecond crystallization steps mentioned above, leading to three or morecrystallization steps.

Besides their temperature ranges, the individual crystallization stepscan also differ in their holding times.

By performing crystallization steps within the above specifiedtemperature ranges, a glass-ceramic material can be achieved comprisingdifferent crystalline phases, such as lithium disilicate, lithiummetasilicate, lithium phosphate, lithium aluminosilicate asbeta-spodumene solid solution, and cristobalite, and that the respectiveproportions of the crystalline phases can be adjusted to the actualneeds.

As will be shown in detail below, it is particularly preferred that thefinal glass-ceramic material comprises as the two main crystallinephases a lithium disilicate phase and a lithium aluminosilicate phase.As mentioned, the present invention allows for providing variousproportions of these phases by adjusting the temperature and theduration of the crystallization steps.

Regarding the co-existence of a lithium disilicate phase and a lithiumaluminosilicate phase, desirable new properties can be achieved. Inparticular, a glass-ceramic body can be achieved having mechanicalproperties between the properties of lithium aluminosilicateglass-ceramic having excellent thermal properties and relativelymoderate mechanical properties (with a flexural strength of 75 to 150MPa and a fracture toughness K_(1c) of 1 to 2 MPa·m^(1/2)) and oflithium disilicate glass-ceramic having a high strength (with a flexuralstrength of 350 to 400 MPa and a fracture toughness K_(1c) of 2.3 to 2.9MPa·m^(1/2)) and relatively low thermal properties (with a coefficientof thermal expansion of 80 to 120·10⁻⁷ deg⁻¹).

For the particular embodiment specified in the examples below it has forexample been shown that the formation of lithium metasilicate andlithium disilicate is favoured at the temperature range from 620° C. to820° C. and glass-ceramic materials having a flexural strength of 300 to400 MPa and a fracture toughness K_(1c) of 2.0 to 2.6 MPa·m^(1/2) with acoefficient of thermal expansion of 60 to 90·10⁻⁷ deg⁻¹ can be achieved.At the temperature range from 825° C. to 860° C., the crystallization oflithium aluminosilicate phases is dominant and a glass-ceramic materialhaving a flexural strength of 280 to 330 MPa and a fracture toughnessK_(1c) of 2.0 to 2.3 MPa·m^(1/2) with a coefficient of thermal expansionof 40 to 60·10⁻⁷ deg⁻¹ can be achieved.

Also, the chemical resistance of the final glass-ceramic can beadjusted. In this regard, a high proportion of lithium disilicatecrystalline phase is generally preferable if a high chemical resistanceof the final glass-ceramic material is to be achieved.

According to a particularly preferred embodiment of the presentinvention, a first region of the glass body is subjected to the firstcrystallization step and a second region of the glass body different tothe first region is subjected to the second crystallization step suchthat the proportion of the first crystalline phase (e.g. lithiumdisilicate) is higher in the first region than in the second region andthe proportion of the second crystalline phase (e.g. lithiumaluminosilicates) is higher in the second region than in the firstregion.

The term “proportion” of the respective crystalline phase is in thecontext of the present invention to be understood as volume-% based onthe total volume of the final glass-ceramic body.

As for the crystallization steps, embodiments are encompassed in whichthe first region is exclusively subjected to the first crystallizationstep and the second region is exclusively subjected to the secondcrystallization step. In particular if the second crystallization stepis at a higher temperature range, it is also thinkable that the secondregion is also subjected to the first crystallization step before it issubjected to the second crystallization step.

The first temperature range is preferably from 620° C. to 820° C. Thesecond temperature range is preferably starting from 825° C., and ismore preferably from 825° to about 1000° C., most preferably from 825°C. to about 860° C.

More particularly, a crystalline phase of Li₂Si₂O₅ (lithium disilicate)is predominantly formed at the first region, and a further crystallinephase being selected from the group consisting of LiAlSi₂O₆, LiAlSiO₄,LiAlSi₃O₈, LiAlSi₄O₁₀ (lithium aluminosilicate) is predominantly formedat the second region, whereby—according to a particularly preferredembodiment—the proportion of the crystalline phases change from oneregion to the other in a gradual manner.

This is of particular relevance for preparing a glass-ceramic materialto be used for dental restorations, since regions comprising a lithiumdisilicate crystalline phase generally are translucent resembling enamelwhereas regions comprising a lithium aluminosilicate crystalline phaseare opaque resembling dentin. By performing a controlled thermaltreatment, a glass-ceramic body can thus be obtained comprisinginhomogenously distributed crystalline phases attributing to aninhomogenous colour distribution resembling the colour distribution of anatural tooth.

Also with regard to the mechanical properties, the structure of anatural tooth can be accurately mimicked by the inhomogenouslydistributed crystalline phases, as the lithium disilicate phase impartsa higher strength than the lithium aluminosilicate phase correspondingto the natural situation with the enamel having a higher strength thanthe dentin.

Also the high chemical resistance of the lithium disilicateglass-ceramic region attributes to its feasibility for an enamel-likeregion.

In view of the CAD/CAM machining of the glass-ceramic body, adjustmentof the formation of different crystalline phases in different regions ofthe body further allows a favourable distribution of stresses whichstrengthens the body and makes it less prone to fracture. In particular,a favourable stress distribution is obtained if the crystalline phaseschange from one region to another in a gradual manner. Regarding e.g.the coefficient of thermal expansion, which is different in a lithiumdisilicate glass-ceramic region than in a lithium aluminosilicateglass-ceramic region, a smooth transition can thus be achieved. Thisultimately leads to a body being well suited for CAD/CAM machining ofboth single-unit restorations as well as multi-unit bridges. Inaddition, the distribution of the crystalline phases can be adjustedsuch that the areas to be machined off the block are preferablypredominantly of a softer material than e.g. the areas which will bepresent in the ultimate restoration.

The process of the present invention is not confined locally to thesurface area of the body, but allows the selective and controlledformation of different crystalline phases throughout the body and inparticular in the interior of the body remote from the surface. Aspatially confined and controlled thermal treatment leading to aselective and controlled crystallization can for example be performed bylaser irradiation, as demonstrated by Kawasaki et al. (Journal ofNon-Crystalline Solids 325 (2003) 61 to 69), Honma et al. (AppliedPhysics Letters 83 (2003), 2796 to 2798), Fujiwara et al. (Chem. Glasses43C (2002) 213), Gupta et al. (Optical Materials 2005) and others. Othermethods allowing a focused and spatially limited heating of the basicglass body, using e.g. electromagnetic radiation or susceptors, are alsopossible. Also, methods using a cooling paste for heat confinement, i.e.by shielding certain areas of the basic glass body from being heated,can likewise be performed.

Although the present invention encompasses also the possibility offorming different crystalline phases in spatially separated regions, theprocess of the present invention is preferably carried out in a mannersuch that different crystalline phases changing from one region toanother in a gradual manner are formed. This is also of particularrelevance concerning the use of the body for a dental restoration asalso in a natural tooth the different structural components change in agradual manner. Also, any stability problems which might arise at aninterface of different materials can be avoided.

According to a very straightforward technique, a temperature gradientcan be achieved in the basic glass body by appropriately placing thebody in a heating furnace in which such a temperature gradient ispresent, thus leading to a gradual change of the crystalline phasecomposition along the gradient.

Apart from the process described above, the present invention furtherrelates to a glass composition comprising 65 to 72 wt-% SiO₂, at least10.1 wt-% Li₂O and at least 10.1 wt-% Al₂O₃ based on the total weight ofthe composition. Preferably, the proportion of Li₂O to Al₂O₃ is at least1:1, and more preferably is about 3:2. It is understood that the term“proportion of Li₂O to Al₂O₃” means the ratio of the amount of Li₂O tothe amount of Al₂O₃.

The glass composition is particularly useful for the process describedabove. Based on this composition, the desirable glass-ceramic materialcan be prepared in a simple and straightforward manner, as mentionedabove. Particularly, a great variety of different crystalline phases maybe formed.

Preferably, the glass compositions of the instant invention comprise atmost 15 wt-% of Li₂O and/or at most 15 wt-% of Al₂O₃.

According to a specific embodiment, the proportion of Li₂O to Al₂O₃ isfrom 1:1 to 1.5:1.

According to a preferred embodiment, the composition further comprises 0to 2 wt-% K₂O, 1 to 4 wt-% Na₂O and 0 to 1.5 wt-% CeO₂. In this regard,the present invention also encompasses a composition essentiallyconsisting of 0 to 2 wt-% K₂O, 1 to 4 wt-% Na₂O and 0 to 1.5 wt-% CeO₂besides SiO₂, Li₂O and Al₂O₃.

Depending on the final glass-ceramic body to be achieved, differentcrystallization agents can be used in the glass composition. Typically,the composition thus further comprises 0 to 1.5 wt-% CaO, 0 to 1.0 wt-%MgO, 0 to 1.5 wt-% B₂O₃, 1 to 5 wt-% P₂O₅, 0 to 3 wt-% CaF₂, 0 to 2.0wt-% AlF₃, 0 to 1.0 wt-% Ag, 0 to 5 wt-% ZrO₂ and 0 to 4 wt-% TiO₂ basedon the total weight of the composition. In this regard, the presentinvention also encompasses a composition essentially consisting of 0 to1.5 wt-% CaO, 0 to 1.0 wt-% MgO, 0 to 1.5 wt-% B₂O₃, 1 to 5 wt-% P₂O₅, 0to 3 wt-% CaF₂, 0 to 2.0 wt-% AlF₃, 0 to 1.0 wt-% Ag, 0 to 5 wt-% ZrO₂and 0 to 4 wt-% TiO₂ besides SiO₂, Li₂O and Al₂O₃ and, optionally, K₂O,Na₂O and CeO₂ in the amounts specified above, whereby preferably theproportion of Li₂O to Al₂O₃ is from 1:1 to 1.5:1. A glass compositionbeing devoid of ZrO₂ and TiO₂ is particularly preferred for achieving arelatively high content of a lithium disilicate crystalline phase, inparticular in comparison to lithium aluminosilicate phases.

According to an alternative preferred embodiment, the glass composition,besides SiO₂, Li₂O and Al₂O₃, comprises 0 to 2 wt-%, preferably 0 to 1wt-% K₂O, at most 4 wt-%, preferably at most 2.5 wt-% Na₂O, 0 to 1.5wt-% CaO, 0 to 1.5 wt-% CeO₂, 1 to 5 wt-%, preferably 3 to 5 wt-% P₂O₅,0 to 0.5 wt-%, preferably 0 to 0.1 wt-%, very preferably 0 to 0.05 wt-%V₂O₅, 0 to 1 wt-% Ag and 0 to 1 wt-% ZrO₂, the composition being devoidof TiO₂, Cu₂O, BaO, Sb₂O₃, Nb₂O₅, MgO, La₂O₃ and SnO₂. It is therebyparticularly preferred that the glass composition essentially consistsof 0 to 2 wt-%, preferably 0 to 1 wt-% K₂O, at most 4 wt-%, preferablyat most 2.5 wt-% Na₂O, 0 to 1.5 wt-% CaO, 0 to 1.5 wt-% CeO₂, 1 to 5wt-%, preferably 3 to 5 wt-% P₂O₅, 0 to 0.5 wt-%, preferably 0 to 0.1wt-%, very preferably 0 to 0.05 wt-% V₂O₅, 0 to 1 wt-%, preferably 0wt-% Ag and 0 to 1 wt-% ZrO₂ besides SiO₂, Li₂O and Al₂O₃ in the amountsgiven above, whereby preferably the proportion of Li₂O to Al₂O₃ is from1:1 to 1.5:1. Also in this alternative embodiment, the glass compositionis preferably devoid of ZrO₂ for achieving a relatively high content ofa lithium disilicate crystalline phase, in particular in comparison tolithium aluminosilicate phases.

According to another alternative preferred embodiment, the glasscomposition comprises 65 to 72 wt-% SiO₂, at least 10.1 wt-% Li₂O, atleast 10.1 wt-% Al₂O₃, whereby preferably the proportion of Li₂O toAl₂O₃ is from 1:1 to 1.5:1, 1 to 5 wt-%, preferably 3 to 5 wt-% P₂O₅,and optionally 0 to 1.5 wt-% CeO₂, 0 to 0.1 wt-%, preferably 0 to 0.05wt-% V₂O₅, 0 to 2 wt-%, preferably 0 to 1 wt-% K₂O, at most 4 wt-%,preferably at most 2.5 wt-% Na₂O, 0 to 1.5 wt-% CaO, 0 to 1 wt-% Ag and0 to 1 wt-% ZrO₂, the composition being devoid of TiO₂, Cu₂O, BaO,Sb₂O₃, Nb₂O₅, MgO, La₂O₃ and SnO₂. It is thereby particularly preferredthat the glass composition essentially consists of 65 to 72 wt-% SiO₂,at least 10.1 wt-% Li₂O, at least 10.1 wt-% Al₂O₃, whereby preferablythe proportion of Li₂O to Al₂O₃ is from 1:1 to 1.5:1, 1 to 5 wt-%,preferably 3 to 5 wt-% P₂O₅, and optionally 0 to 1.5 wt-% CeO₂, 0 to 0.1wt-%, preferably 0 to 0.05 wt-% V₂O₅, 0 to 2 wt-%, preferably 0 to 1wt-% K₂O, at most 4 wt-%, preferably at most 2.5 wt-% Na₂O, 0 to 1.5wt-% CaO, 0 to 1 wt-% Ag and 0 to 1 wt-% ZrO₂. Also in this alternativeembodiment, the glass composition is preferably devoid of ZrO₂ forachieving a relatively high content of a lithium disilicate crystallinephase, in particular in comparison to lithium aluminosilicate phases.

Typical glass compositions suitable for the purposes of the presentinvention are as follows:

Composition I component amount (wt-%) SiO₂ 67.3 Li₂O 10.1 Al₂O₃ 10.1 K₂O0.5 Na₂O 2.5 ZrO₂ 3.0 CeO₂ 1.4 V₂O₅ 0.05 CaO 1.55 P₂O₅ 3.5

Composition II component amount (wt-%) SiO₂ 68.7 Li₂O 10.1 Al₂O₃ 10.1K₂O 0.5 Na₂O 2.5 ZrO₂ 0.5 CeO₂ 1.5 V₂O₅ 0.05 CaO 1.55 P₂O₅ 4.5

Composition III component amount (wt-%) SiO₂ 68.0 Li₂O 10.5 Al₂O₃ 10.5K₂O 0.5 Na₂O 2.5 ZrO₂ 0.5 CeO₂ 1.5 V₂O₅ 0.05 CaO 1.45 P₂O₅ 4.5

Composition IV component amount (wt-%) SiO₂ 69.5 Li₂O 10.5 Al₂O₃ 10.5K₂O 0.5 Na₂O 1.0 ZrO₂ 0.5 CeO₂ 1.5 V₂O₅ 0.05 CaO 1.45 P₂O₅ 4.5

Composition V component amount (wt-%) SiO₂ 69.5 Li₂O 15.0 Al₂O₃ 10.5CeO₂ 1.45 V₂O₅ 0.05 P₂O₅ 3.5

All preferred features of the glass composition mentioned above, and inparticular all specific glass compositions, are analogously preferredwith regard to the described process for preparing a glass-ceramic body.They are likewise preferred with regard to the glass-ceramic body itselfand to its use for a dental restoration, described below.

According to a further aspect, the present invention further relates aglass-ceramic body comprising at least two crystalline phases selectedfrom the group consisting of Li₂SiO₃, Li₂Si₂O₅, LiAlSi₂O₆, LiAlSiO₄,LiAlSi₃O₈, LiAlSi₄O₁₀ and Li₃PO₄.

Preferably, the glass-ceramic body comprises five crystalline phasesselected from the group consisting of Li₂SiO₃, Li₂Si₂O₅, LiAlSi₂O₆,LiAlSiO₄, LiAlSi₃O₈, LiAlSi₄O₁₀ and Li₃PO₄, thus allowing an almostunlimited adjustment of properties of the body by suitably choosing thecrystalline phases and the proportion in which they are present.

According to a particularly preferred embodiment, the glass-ceramic bodycomprises

-   a) a first crystalline phase of Li₂Si₂O₅ (lithium disilicate) and-   b) a second crystalline phase selected from the group consisting of    LiAlSi₂O₆, LiAlSiO₄, LiAlSi₃O₈ and LiAlSi₄O₁₀.

The second crystalline phase is thus a lithium aluminosilicate; amongthe group mentioned, LiAlSi₂O₆ and LiAlSi₃O₈ are preferred.

A particularly preferred combination is LiAlSi₂O₆, LiAlSi₃O₈ andLi₂Si₂O₅ further comprising Li₃PO₄.

As mentioned above, the type of crystalline phase as well as itsproportion in the glass-ceramic body of the present invention can becontrolled by adjusting the temperature profile. For example, a twophase material having a ratio of lithium aluminosilicate and lithiumdisilicate ranging from about 30:70 to about 60:40 can be achieved.

As also mentioned above, it is preferred that the glass-ceramic bodycomprises a first region and a second region different to the firstregion, wherein in the first region the proportion of the firstcrystalline phase is higher than in the second region and in the secondregion the proportion of the second crystalline phase is higher than inthe first region, thus allowing the inhomogeneous structure of e.g. anatural tooth to be mimicked.

This includes embodiments comprising in the first and the second regionat least one further crystalline phase in addition to the firstcrystalline phase and the second crystalline phase, respectively. Inparticular, it encompasses embodiments comprising in the first regionalso the second crystallization phase to a lower proportion than thefirst crystalline phase and/or in the second region also the firstcrystalline phase to a lower proportion than the second crystallinephase.

In this regard, it is further preferred that the first crystalline phaseand the second crystalline phase change gradually from region to region.This encompasses embodiments in which the first crystalline phasedecreases gradually with an increase in the second crystalline phase andvice versa, i.e. without a purely amorphous phase disposed between theregions, as well as embodiments in which the first crystalline phasedecreases in direction to the second region and the second crystallinephase decreases in direction to the first region with a purely amorphousphase disposed between the regions.

According to a particularly preferred embodiment, the glass-ceramic bodyis in the form of a dental restoration having an enamel area and adentin area corresponding to the respective areas of a natural tooth,with the first region being arranged in the enamel area and the secondregion being arranged in the dentin area.

Since with regard to colour and mechanical properties, the lithiumdisilicate phase resembles the enamel, whereas the lithiumaluminosilicate phase resembles the dentin of a natural tooth, superiorrestorations can be achieved by suitable choosing the distribution ofthese phases. Also, due to the lithium aluminosilicate being opaque, ametal abutment or implant can be shielded from shining through therestoration by suitably arranging the lithium aluminosilicate phasewithin the body.

According to a further aspect, the present invention thus also relatesto the use of the glass-ceramic body for a dental restoration.Specifically, the present invention relates to the use of theglass-ceramic body for inlays, onlays, veneers, crowns and bridges up tomulti-unit bridges.

In particular, this use also encompasses the use of the glass-ceramicbody as a blank for a CAD/CAM machining process to prepare a dentalrestoration. Likewise, the basic glass body can be subjected to theCAD/CAM machining process prior to the thermal treatment involving thecrystallization steps, since the change in volume of the bodyaccompanied by the formation of the crystalline phases is insignificant.

It is understood that the glass-ceramic body of the present inventioncan likewise be used for other technological areas, in particular areasin which a good heat shock resistance and/or chemical resistance of thematerial is of relevance.

Specifically the glass-ceramic body can be used for cooktops (plates andother elements), cookware and/or bakeware. A particularly interestinguse is in the field of (chemical) laboratory equipment, which in generalis both subjected to high temperatures as well as to harsh chemicalconditions.

Other areas include apparatuses for the generation, the distribution andthe use of energy, in particular power plants. A specific area ofinterest is the use in solar heat collection elements comprising aglass-ceramic central tube.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is further illustrated by way of the followingexamples in combination with the attached figures, of which

FIG. 1 shows a graphical representation of the proportion of differentphases (in volume-%) in a glass-ceramic material obtained by subjectingthe glass composition according to the present invention to differenttemperature treatments; and

FIG. 2 shows a purely schematic representation of a preferredglass-ceramic body according to the present invention to be subjected toa CAD/CAM process for preparing a dental restoration, as well as aholder for holding the body.

DETAILED DESCRIPTION Examples

The following experiments are based on the following (raw) glasscomposition:

component amount (wt-%) SiO₂ 66.5 Li₂O 10.5 Al₂O₃ 10.5 K₂O 0.45 Na₂O 4.0ZrO₂ 3.0 CeO₂ 1.5 V₂O₅ 0.05 P₂O₅ 3.5

Differential Scanning Calorimetry (DSC) and Differential ThermalAnalysis (DTA) of the composition has shown three peaks, one at about655° C., one at about 812° C. and one at about 826° C., indicative ofthree crystallization steps.

Based on these findings, a first sample of the glass-compositionhas—after a nucleation step at 550° C. for three hours—been subjected toa crystallisation step at 660° C. for three hours (crystallisation stepI). A second and a third sample were subjected to a crystallisation stepat 815° C. for three hours (crystallisation step II) followingcrystallisation step I and a crystallisation step at 830° C. for threehours (crystallisation step III) following crystallisation step I.

X-ray diffraction (XRD) analysis has revealed a formation of Li₂SiO₃(lithium metasilicate) and lithium aluminosilicate (LAS) atcrystallization step I, and a formation of Li₂Si₂O₅ (lithium disilicate)and lithium aluminosilicate at crystallization step II andcrystallization step III, with an increased content of lithiumaluminosilicate (as spodumen) and a decreased content of lithiumdisilicate formed in crystallization step III in comparison tocrystallization step II.

The content of different phases in the final glass-ceramic in relationto different heat treatments has further been determined. In thisregard, the raw glass composition has—after a nucleation step at 550° C.for three hours and a first crystallization step at 660° C. for threehours—been subjected to a second crystallization step at a furthertemperature for three hours, specifically at a temperature of 760° C.(sample 1), 790° C. (sample 2), 820° C. (sample 3) and 850° C. (sample4). The results are shown in FIG. 1.

As shown in FIG. 1, the content of the different phases in the finalglass-ceramic material is highly dependent on the temperature of thesecond crystallization step. For example, a decrease in the amorphousphase with an increase in the temperature of the second crystallizationstep has been detected. For the lithium disilicate phase, the highestcontent has been detected in samples 2 and 3, for which the secondcrystallization step has been at a temperature of 790° C. and 820° C.,respectively. Lithium aluminosilicate is in sample 1 predominantlypresent as petalite and in sample 2 almost exclusively present asvirgilite. In sample 3, it is present both as virgilite and spodumene,whereas in sample 4 it is exclusively present as spodumene.

The results given in FIG. 1 both illustrate that several crystallinephases can be formed in one and the same glass-ceramic material and thatthe type of crystalline phase and its content can be controlled byadjusting the temperature treatment.

It has been shown that different crystalline phases resulting indifferent mechanical and optical properties can be achieved in one andthe same glass-ceramic body by applying a temperature gradient for theheat treatment. For example, a temperature gradient can be provided in afurnace in which the temperature gradually decreases with increasingdistance from the heating source of the furnace (e.g. located in themiddle of the furnace). By appropriately placing the respective bodyinto the furnace, the temperature gradient is established in thematerial, leading to crystalline phases gradually changing from oneregion to another.

Specifically, it has been shown that by subjecting the glass compositionof the present example to a temperature gradient starting at about 550°C., opalescence starts to form at about 570° C. At about 620° C., aviolet shade in reflectance light and a yellow shade in transmittancelight can be detected, and at about 670° C. opalescence is marked. Anopaque material is achieved starting at about 700° C.

By means of the glass composition of the present example it could, thus,be shown that the invention not only allows for the formation ofdifferent crystalline phases in different regions of one and the samebody, but also for a gradual change of the crystalline phases from oneregion to another.

As schematically shown in FIG. 2, the glass-ceramic body 2 of thepresent invention comprises a first region 4 comprising a highproportion of a first crystalline phase and a second region 6 comprisinga high proportion of a second crystalline phase. Depending on the localproperties to be achieved in the final restorations 8, the portions tobe removed are determined and the body is arranged correspondingly. Aholder 10 safeguards that the body is kept in place during thecomputer-aided machining.

Given the distribution of the crystalline phases, a final restorationcan be achieved, the load bearing surfaces 12 having a higher toughnessthan e.g. the bulk area 14 of the body 2 to be removed. Thus, a dentalrestoration with high toughness in e.g. the pontics, the cusp supportingareas or the edges can be achieved in a relatively easy manner withoutundue wear of the machining tools.

The invention claimed is:
 1. A process for preparing a glass-ceramicbody comprising the steps of providing a basic glass body and subjectingthe basic glass body to a thermal treatment whereby a crystalline phaseembedded in a glass matrix is formed, wherein the basic glass body ismade of a composition comprising 65 to 72 wt-% SiO₂, at least 10.1 wt-%Li₂O and at least 10.1 wt-% Al₂O₃ based on the total weight of thecomposition, the proportion of Li₂O to Al₂O₃ being from 1:1 to 1.5:1,and the thermal treatment involves a nucleation step followed by a firstcrystallization step at a first temperature range and a secondcrystallization step at a second temperature range different from thefirst temperature range, wherein at least two different crystallinephases are formed.
 2. The process for preparing a glass-ceramic bodyaccording to claim 1, wherein a first region of the glass body issubjected to the first crystallization step and a second region of theglass body different to the first region is subjected to the secondcrystallization step such that the proportion of the first crystallinephase is higher in the first region than in the second region and theproportion of the second crystalline phase is higher in the secondregion than in the first region.
 3. The process according to claim 2,wherein the regions are heated to the respective temperature ranges bymeans of laser irradiation, electromagnetic radiation and/or susceptors.4. The process according to claim 1, wherein the first temperature rangeis from 620 to 820° C. and the second temperature range is starting from825° C.
 5. A glass composition comprising: 65 to 72 wt-% SiO₂, at least10.1 wt-% Li₂O, at least 10.1 wt-% Al₂O₃, 0 to 2 wt-% K₂O, 1 to 4 wt-%Na₂O, 0 to 1.5 wt-% CaO, 0 to 1.0 wt-% MgO, 0 to 1.5 wt-% B₂O₃, 0 to 1.5wt-% CeO₂, 1 to 5 wt-% P₂O₅, 0 to 3 wt-% CaF₂, 0 to 2.0 wt-% AlF₃, 0 to1.0 wt-% Ag, 0 to 5 wt-% ZrO₂, and 0 to 4 wt-% TiO₂ based on a totalweight of the composition, the proportion of Li₂O to Al₂O₃ being from1:1 to 1.5:1.
 6. The glass composition according to claim 5, comprisingat most 15 wt-% of Li₂O and/or at most 15 wt-% of Al₂O₃.
 7. Aglass-ceramic body comprising: a) a first crystalline phase of Li₂Si₂O₅,and b) a second crystalline phase selected from the group consisting ofLiAlSi₂O₆, LiAlSiO₄, LiAlSi₃O₈ and LiAlSi₄O₁₀; the glass-ceramic bodycomprising a first region and a second region different from the firstregion; wherein: a proportion of the first crystalline phase is higherin the first region than in the second region; and a proportion of thesecond crystalline phase is higher in the second region than in thefirst region.
 8. The glass-ceramic body according to claim 7, wherein:the proportion of the first crystalline phase decreases and theproportion of the second crystalline phase increases in a direction fromthe first region to the second region; or the proportion of the firstcrystalline phase decreases in a direction from the first region to thesecond region and the proportion of the second crystalline phasedecreases in a direction from the second region to the first region withan amorphous phase disposed between the first region and the secondregion.
 9. The glass-ceramic according to claim 7, the glass-ceramicbody being in the form of a dental restoration having an enamel area anda dentin area corresponding to the respective areas of a natural tooth,with the first region being arranged in the enamel area and the secondregion being arranged in the dentin area.
 10. A method of forming adental restoration comprising forming a dental restoration from theglass-ceramic body of claim
 7. 11. The glass-ceramic body according toclaim 8, the glass-ceramic body being in the form of a dentalrestoration having an enamel area and a dentin area corresponding to therespective areas of a natural tooth, with the first region beingarranged in the enamel area and the second region being arranged in thedentin area.